1875 R. Hunt & F. W. Rudler (ed. 7) II. 906 The compounds [of indigo-sulphuric or sulphindigotic acid] are called indigo-sulphates, and are, like the acid, of a dark blue colour... The indigo-sulphates of the alkalis may..be prepared by steeping wool, previously well cleaned, into the solution in sulphuric acid.